Industrial separations of normally gaseous alkenes from alkanes and separations of alkenes from other non-hydrocarbon gases by traditional methods such as distillation can be challenging due to their low and often similar boiling points. Notably challenging are separations of compounds having the same number of carbon atoms, such as ethylene from ethane, propylene from propane, and butane from butane. Similar boiling points necessitate large-scale capital-intensive facilities and high energy input for effective separation, which may involve cryogenic distillation conditions. Membrane separation processes may be less expensive and require significantly less energy. Thin-film composite membranes that incorporate ionomers may be used for facilitated transport of alkenes.
Membranes comprising gas-separation layers of silver salts of certain ionomer containing polymers, especially fluorinated polymers, have been shown to be useful for the separation of alkenes from alkanes or from other non-hydrocarbon gases. One might assume that at a minimum a membrane consisting solely of gas-separation layer of a silver ionomer would be useful. However, fluorinated ionomers incorporating silver can be costly and the alkene flux, which is the rate at which the alkene permeates through the membrane per unit area, would be impractically low for a necessarily thick membrane that is mechanically strong. A membrane composite can overcome these limitations and usually comprises a thin gas-separation layer and other layers of dissimilar materials contacted together in combination to form a single composite construction that optimize and improve the composite membrane performance and durability. The use of various membranes and certain reinforced composites comprising a gas-separation layer of a silver ionomer for facilitated transport of alkenes and the separation of alkenes from alkanes have been described. See for example A. van Zyl, et al. in Journal of Membrane Science 1997 133 15-26, 0. I. Eriksen, et al. in Journal of Membrane Science 1993 85 89-97, A. van Zyl Journal of Membrane Science 1997 137 175-185, and U.S. Pat. No. 5,191,151.
Eriksen et al. disclosed composite membranes for the separation of alkenes from alkanes in the third embodiment of the invention in U.S. Pat. No. 5,191,151. The composite membranes comprise gas separation materials of perfluorinated ionomers that are copolymers of tetrafluoroethylene and a perfluorovinyl ether containing a terminal sulfonic acid group, such as Nafion® (Chemours, Wilmington Del.). The sulfonic-acid ionomers as 5% solutions in lower alcohols/water were modified by mixing with at least one silver compound under such conditions to obtain a solution comprising a silver-exchanged copolymer (and also the conjugate acid of the anion from the silver compound). Highly soluble and “ionizable” silver compounds such as AgBF4, AgClO4, and AgNO3 (preferred) were used at ratios of g-equivalents of silver to g-equivalents of sulfonic-acid groups (present in the ionomer) in the range of 0.5:1 to 50:1, preferably at about 1:1 to stoichiometric excesses of about 40:1. Membranes with thicknesses from 0.1 to 400-μm were disclosed and composite membranes I and J, having 20 and 30-μm thicknesses, respectively, were enabled in Example 5 by casting onto porous-layer substrates having 0.2-μm average pore diameters. The membranes were presumably interpenetrating composites due to the relatively large pores of the substrate.
As taught by Eriksen et al., we attempted to fabricate laminar composite membranes with significantly (10×) thinner (≤2-μm) gas-separation layers for anticipated commercially attractive alkene permeance and lower costs. Ionomer solutions were prepared by dilution of commercially available Nafion® D2020, with added dissolved silver nitrate, and membranes were cast onto a polyvinylidine fluoride porous-layer support having order-of-magnitude smaller pore diameters of approximately 0.02-μm. Gas testing indicated that membranes were laminar composites, but many were defective and had low or no alkene over alkane selectivity due to suspected “pinholes.” The defective membranes had excessively high helium permeability that was likely due to incomplete pore-bridging of the support and a mechanically fragile gas-separation layer. We recognized that membranes cast from mixtures of ionomer and dissolved ionic compounds, not associated with the ionomer, can be inherently fragile due to hindered and incomplete film coalesce. See for example S. D. Minteer et al. in Journal of Membrane Science 2003 213 55-66. Such prepared membranes are not very mechanically durable and cannot be easily and consistently fabricated defect-free into larger, more complex, and commercially relevant geometries such as spiral-wound membrane modules.